1. Field of Invention
The present invention relates to producing a scale for use in opto-electronic scale reading apparatus.
2. Description of Related Art
A known form of opto-electronic scale reading apparatus for measuring relative displacement of two members comprises a scale on one of the members, having scale marks defining a periodic pattern, a read head provided on the other member, means for illuminating the scale, periodic diffraction means for interacting with light from the scale marks to produce interference fringes having movement relative to the read head and detecting means in the read head responsive to the fringes to produce a measure of the displacement.
An example of such apparatus is disclosed in EP-A-0 207 121 and also U.S. Pat. No. 4,974,962, each of which shows the means for illuminating and the periodic diffraction means in the read head. U.S. Pat. No. 4,926,566 discloses a method of producing a scale, in the form of a flexible tape produced by rolling, the pitch of the scale marks being 20 xcexcm or 40 xcexcm for example. The illuminating means, the diffraction means and the detecting means responsive to the fringes may be integrated in the read head in the manner described in U.S. Pat. No. 5,302,820. Such a scale may have reference marks along its length, as disclosed in WO94/25830 in which a depression is provided on the scale by stamping the scale at appropriate intervals with a cylindrical roller.
In the production of a length of scale, for example as described in U.S. Pat. No. 4,926,566, although flanks of the embossed profile of the scale are well defined, the crests can be somewhat irregular. Although this is not a problem in scale reading apparatus in which the scale is lit from an end, since the crests are not read, this could be a problem in apparatus in which the scale is lit from one side, in which the crests are read.
According to a first aspect of the present invention, there is provided a method of producing a length of scale for use in opto-electronic scale reading apparatus, comprising subjecting a length of scale to an embossing process to form a profile which is a sequence of troughs and crests, wherein after the embossing process the length of scale is subjected to a further process which acts on the crests to make more regular their shapes.
The further process could comprise passing the embossed length of scale between first and second rollers to render the crests more regular as regards their shapes. In this case, one of the rollers could be a driven roller to pass the length of scale between the rollers. Alternatively, the length of scale could be pulled through the rollers. Each of the rollers could be a plain roller.
In a second aspect of the invention, there is provided a method of providing reference marks or identification marks on the scale during the embossing process or the process which acts on the scale to make their shapes more regular.
The reference marks or identification marks may be imprinted on the scale in either process by providing the relevant roller with suitable marks.